Process for the purification of thiophenes

ABSTRACT

The invention relates to a process for the purification of thiophenes by means of precipitation. The purified thiophenes are liquid at room temperature, have a purity of at least 99.50 wt. %, and are represented by the following general formula (I),  
                 
 
wherein R 1  and R 2  independently of each other are, for example, a linear or branched C 1 -C 20 -alkyl group, or together form a fused C 1 -C 20 -dioxyalylene ring. The process involves: (I) precipitating the thiophene by cooling a solution of the thiophene and at least one solvent; or (II) precipitating the thiophene by adding the thiophene to a cooled solution of solvent and optionally the thiophene. The solutions are cooled to a temperature below the melting point of the thiophene.

CROSS REFERENCE TO RELATED PATENT APPLICATION

The present patent application claims the right of priority under 35U.S.C. §119 (a)-(d) of German Patent Application No. 103 43 873.4, filedSep. 23, 2003.

FIELD OF THE INVENTION

The invention relates to a process for the purification of thiopheneswhich are liquid at room temperature, the thiophenes purified by thisprocess and their use.

BACKGROUND OF THE INVENTION

Thiophenes are used, for example, for the preparation of conductivepolymers. Poly(3,4-alkylenedioxythiophenes) such as are described, forexample, in EP-A 339 340, are of particular interest in this context.These compounds are distinguished by particular properties, such as highconductivity, high transparency and outstanding long-term stability.They have therefore found increasing use in industry as organicconductive polymers. Thus e.g. through-plating of printed circuitboards, antistatic treatment of photographic films and use as anelectrode or solid electrolyte in solid electrolyte capacitors aredescribed as important fields of use.

An important prerequisite in the preparation of organic conductivepolymers is high purity of the starting substances needed for theirpreparation. Impurities contained in the starting substance canadversely influence the polymerization in that the polymerization doesnot take place, or takes-place only very slowly or incompletely, or isaccelerated to an uncontrolled extent. The processing time of thesemonomers can consequently drop drastically, so that these can no longerbe employed in the processing processes.

In addition, the properties of the resulting polymers may also beadversely influenced in that the impurities, for example, adverselychange the intrinsic colour of the resulting polymer and as a result thetransparency, which is essential for the use of the polymers e.g. astransparent conductive or antistatic coatings, is impaired.

Impurities which are also capable of polymerization can beco-incorporated into the polymer and thereby significantly lower theconductivity thereof. Further adverse effects of impurities can be thatthe order of the conductive layers may be lowered by impurities, wherebypoorer conductivities result, that impurities become concentrated on thesurface of the polymer after the polymerization and undesirabletransition resistances thereby result, so that the function of theconductive layer is restricted, or that the long-term stability of theconductive polymers is adversely influenced in that the impurities, forexample, initiate reaction of the conductive polymer with oxygen andthus significantly impair the properties of the polymer.

The starting substances which are needed for the preparation of organicconductive polymers and are as a rule prepared from raw materials bychemical reactions, are therefore purified before their use.

A number of purification operations which are in principle suitable forpurification of the monomers for the polymerization to give organicconductive polymers are known to the expert. Such purification methodsare, for example, distillation, sublimation, extraction,crystallization, chromatography and adsorption. These purificationmethods have been known to the expert for a long time and are describedin the usual textbooks.

Thiophenes which are liquid at room temperature and are suitable for thepreparation of electrically conductive polymers are of particularimportance because of their easy processability in the liquid form. Forthe purification of these thiophenes the expert has available thepurification methods which can be used on liquid substances, preferablydistillation, which is also carried out on a large industrial scale,extraction and chromatography.

Distillative purification of thiophenes as monomers for use for thepreparation of electrically conductive polymers is known, for example,from EP-A 1 142 888. The doctrine of EP-A 1 142 888 is that the numberand amount of by-products can be reduced by optimized reactionconditions and e.g. 3,4-ethylenedioxythiophene is obtainable in a purityof up to 97.7%. However, the doctrine of EP-A 1 142 888 furthermore isthat for further purification an additional extraction is necessary inorder to remove water-soluble by-products and to achieve a purity ofmore than 99%. 3,4-Dimethoxythiophene predominantly occurs as asecondary component, i.e. impurity, in this synthesis of3,4-ethylenedioxythiophene.

Furthermore, separating off of compounds by distillation is onlypossible if the components to be separated differ significantly, i.e. bymore than 1° C., in their boiling points. The less the boiling pointsdiffer, the greater the expenditure on apparatus for the separation, sothat such separations are no longer to be carried out economically.Since substituted thiophenes, such as, for example,alkylenedioxythiophenes, are preferably distilled under reducedpressure, the difference in the boiling points is reduced further, whichfurther increases the expenditure on separation.

The purification of 3,4-alkylenedioxythiophenes, in particular of3,4-ethylenedioxythiophene, which are contaminated with3,4-dimethoxythiophene represents a particular difficulty. Thus, forexample, 3,4-dimethoxythiophene produced during the synthesis of3,4-ethylenedioxythiophene can be separated off only with a highexpenditure because of the molecular weight differing by only two unitsand the very similar structure, which makes purification viadistillation no longer economical beyond a certain degree of purity.3,4-Dimethoxythiophene as an impurity has the disadvantage, however,that it is co-incorporated into the polymer during polymerization andcan thus adversely influence properties of the polymer, such as, forexample, the conductivity.

Chromatographic purification of thiophenes as monomers for use for thepreparation of electrically conductive polymers is also known. WO-A02/79295 describes the preparation of liquid and solid chiralalkylenedioxythiophenes and mentions in examples the purification bychromatography on silicon dioxide. The compounds prepared according toWO-A 02/79295 have purities of up to 99.7% after purification. However,chromatographic separation also has disadvantages. Thus, large amount ofsolvents are needed to carry it out, since the compounds to be separatedmust be in a very dilute form in order to achieve the desired separationeffect. Furthermore, the chromatographic separation cannot be operatedcontinuously with the aid of simple apparatuses, so that in each caseonly small amounts of the desired purified thiophene are obtained. Acontinuous separation of large amounts would therefore be associatedwith an extremely high expenditure on apparatus, so that such apurification of thiophenes can no longer be carried out economically.

Conventional recrystallization in which thiophenes which are solid atroom temperature are dissolved at elevated temperature, usually underreflux of the solvent, and are then crystallized out again by cooling isalso known for the purification of thiophenes as monomers for use forthe preparation of electrically conductive polymers and is described inWO-A 02/79295, but is limited to thiophenes which are solid at room tm.

A particular form of crystallization can also be used for thecrystallization of liquid thiophenes. This specific form ofcrystallization, melt crystallization, is described, for example, in N.Wynn, Chem. Engineering (1986), 93(8), 26-27 and in J. Ulrich and H, C.Bülau, Editor(s): Myerson, Allan S. “Handbook of IndustrialCrystallization (2nd Edition)” (2002), 161-179. Melt crystallization issubstantially based on cooling a liquid substance until a melt isformed, from which only the substance to be purified crystallizes out.After crystallization, the mother liquid, which in the ideal casecontains all the impurities, is separated off. Where appropriate, thecrystallized substance is heated gently so that impurities adhering tothe product can be removed together with some of the substance which isthen melting. However, this process is limited to substances orsubstance mixtures which contain relatively large amounts of impuritieswhich can be separated off in liquid form. Small amounts of impuritiescan be removed only uneconomically via this process, since large amountsof the desired compound have to be separated off at the same time inorder to wash out the small amount of impurity. Moreover, meltcrystallization is critical in respect of the temperature programme andtherefore expensive on apparatus.

SUMMARY OF THE INVENTION

There was therefore still a need for a process for the purification ofthiophenes which are liquid at room temperature in which an extremelyhigh purity, preferably of more than 99.9%, is achieved and which doesnot have the disadvantages described above.

The present invention was therefore based on the object of discovering aless expensive process for the purification of thiophenes with whichhighly pure 3,4-alkylenedioxythiophenes, preferably with a purity ofmore than 99.9%, can be prepared.

In accordance with the present invention, there is provided a processfor purifying a thiophene represented by general formula (I),

-   -   wherein,    -   R¹ and R² are each selected independently from the group        consisting of hydrogen,        -   linear or branched, optionally substituted C₁-C₂₀-alkyl            groups, linear or branched C₁-C₂₀-oxyalkyl groups, linear or            branched C₁-C₂₀-oxyalkyl groups which are interrupted by 1            to 5 oxygen atoms, linear or branched C₁-C₂₀-oxyalkyl groups            which are interrupted by 1 to 5 sulphur atoms, a fused ring            of linear or branched, optionally substituted            C₁-C₂₀-dioxyalkylene formed by R¹ and R² together, and a            fused ring of linear or branched, optionally substituted            C₁-C₂₀-dioxyarylene formed by R¹ and R² together, and            said thiophene being liquid at room temperature,            said method being selected from the group consisting of,            a method (I) comprising,    -   (a) providing a first solution comprising said thiophene and at        least one solvent, and    -   (b) cooling said first solution to a temperature below the        melting temperature of said thiophene, thereby precipitating        said thiophene as a solid from said first solution, and        a method (II) comprising,    -   (a) providing a second solution comprising at least one solvent        and optionally said thiophene,    -   (b) cooling said second solution to a temperature below the        melting temperature of said thiophene, to form a cooled second        solution, and    -   (c) adding said thiophene to said cooled second solution,        thereby precipitating said thiophene as a solid from said cooled        second solution.

Unless otherwise indicated, all numbers or expressions, such as thoseexpressing process conditions, etc., used in the specification andclaims are understood as modified in all instances by the term “about.”

DETAILED DESCRIPTION OF THE INVENTION

In the context of the invention, thiophenes which are liquid at roomtemperature are to be understood as those thiophenes which have theirmelting point below +40° C., preferably below +30° C.

In the context of the invention, room temperature can be a temperatureof 10 to 40° C., preferably 15 to 30° C., particularly preferably 18 to25° C.

Thiophenes of the general formula (I) which are preferably purified withthe process according to the invention are compounds of the generalformula (II)

wherein

-   A represents an optionally substituted C₁-C₅-alkylene radical or a    C₁-C₁₂-arylene radical, preferably an optionally substituted    C₂-C₃-alkylene radical,-   R represents a linear or branched, optionally substituted    C₁-C₁₈-alkyl radical, preferably linear or branched, optionally    substituted C₁-C₁₄-alkyl radical, an optionally substituted    C₅-C₁₂-cycloalkyl radical, an optionally substituted C₆-C₁₄-aryl    radical, an optionally substituted C₇-C₁₈-aralkyl radical, an    optionally substituted C₁-C₄-hydroxyalkyl radical, preferably    optionally substituted C₁-C₂-hydroxyalkyl radical, or a hydroxyl    radical,-   x represents an integer from 0 to 8, preferably from 0 to 6,    particularly preferably 0 or 1 and    in the case where several radicals R are bonded to A, these can be    identical or different.

The general formula (II) is to be understood such that the substituent Rcan be bonded to the alkylene or arylene radical A x times.

Preferred compounds of the general formula (II) are those of the generalformula (IIa)

wherein

-   R has the meaning given in the general formula (II) and y represents    0, 1, 2, 3 or 4.

In the context of the invention, C₁-C₅-alkylene radicals A aremethylene, ethylene, n-propylene, n-butylene or n-pentylene. In thecontext of the invention, C₁-C₁₂-arylene radicals can be, for example,phenylene, naphthylene, benzylidene or anthracenylidene. In the contextof the invention, C₁-C₁₈-represents linear or branched C₁-C₁₈-alkylradicals, such as, for example, methyl, ethyl, n- or isopropyl, n-,iso-, sec- or tert-butyl, n-pentyl, 1-methylbutyl, 2-methylbutyl,3-methylbutyl, 1-ethylpropyl, 1,1-dimethylpropyl, 1,2-dimethylpropyl,2,2-dimethylpropyl, n-hexyl, n-heptyl, n-octyl, 2-ethylhexyl, n-nonyl,n-decyl, n-undecyl, n-dodecyl, n-tridecyl, n-tetradecyl, n-hexadecyl orn-octadecyl. C₁-C₂₀-alkyl groups moreover include, for example,n-nonadecyl and n-eicosyl. In the context of the invention,C₅-C₁₂-cycloalkyl represents C₅-C₁₂-cycloalkyl radicals, such as, forexample, cyclopentyl, cyclohexyl, cycloheptyl, cyclooctyl, cyclononyl orcyclodecyl, C₅-C₁₄-aryl represents C₅-C₁₄-aryl radicals, such as, forexample, phenyl or naphthyl, and C₇-C₈-aralkyl represents C₇-C₁₈-aralkylradicals, such as, for example, benzyl, o-, m- or p-tolyl, 2,3-, 2,4-,2,5-, 2,6-, 3,4- or 3,5-xylyl or mesityl. In the context of theinvention, C₁-C₂₀-oxyalkyl represents C₁-C₂₀-oxyalkyl radicals, such as,for example, methoxy, ethoxy, n- or iso-propoxy, n-, iso-, sec- ortert-butoxy, n-pentyloxy, 1-methylbutyloxy, 2-methylbutyloxy,3-methylbutyloxy, 1-ethylpropyloxy, 1,1-dimethylpropyloxy,1,2-dimethylpropyloxy, 2,2-dimethylpropyloxy, n-hexyloxy, n-heptyloxy,n-octyloxy, 2-ethylhexyloxy, n-nonyloxy, n-decyloxy, n-undecyloxy,n-dodecyloxy, n-tridecyloxy, n-tetradecyloxy, n-hexadecyloxy,n-octadecyloxy, n-nonadecyloxy or n-eicosyloxy. The preceding listserves to explain the invention by way of example and is not to beregarded as conclusive. Further substituents of the alkylene or aryleneradicals A which are optionally possible are numerous organic groups,for example alkyl, cycloalkyl, aryl, halogen, ether, thioether,disulfide, sulfoxide, sulfone, sulfonate, amino, aldehyde, keto,carboxylic acid ester, carboxylic acid, carbonate, carboxylate, cyano,alkylsilane and alkoxysilane groups, as well as carboxylamide groups.

If the thiophene to be purified has one or more stereocentres, thethiophene can be a racemate, an enantiomerically pure ordiastereomerically pure compound or an enantiomerically enriched ordiastereomerically enriched compound. An enantiomerically enrichedcompound is to be understood as meaning a compound having an enantiomerexcess (ee) of more than 50%. A diastereomerically enriched compound isto be understood as meaning a compound having a diastereomer excess (de)of more than 30%. According to the invention, however, the compound canalso be any desired mixture of diastereomers.

Before purification with the process according to the invention, thethiophene to be purified preferably has a purity of greater than 70%,particularly preferably a purity of greater than 90%.

The thiophenes of the general formulae (I), (II) or (IIa) to be purifiedcan be prepared by processes known to the expert. Such a preparationprocess is described, for example, in EP-A 1 142 888.

Solvents which are employed are those in which the thiophene to bepurified dissolves and which have a sufficiently low melting point,preferably below −40° C. Examples of suitable solvents which may bementioned are isobutyl methyl ketone, chloroform, methylene chloride,toluene, methanol, propanol, ethanol, acetone, iso-propanol, n-butanol,sec-butanol, dimethylformamide, methyl tert-butyl ether,tetrahydrofuran, diethyl ether, hexane or pentane.

Preferred solvents are polar solvents, and alcohols are particularlypreferred in this context. Methanol or ethanol are very particularlypreferred.

The solvent can also be a mixture of two or more solvents.

Mixtures of one or more alcohol(s) optionally with one or more furthersolvent(s) are preferred in this context. For this purpose it is notabsolutely necessary for each individual solvent to dissolve thethiophene and to have a correspondingly low melting point, merely themixture must have these properties. A mixture of two alcohols isparticularly preferred, and a mixture of methanol and ethanol is veryparticularly preferred.

The solvent is mixed with the thiophene in a ratio of 0.01:1 to 10:1,preferably in a ratio of 0.3:1 to 3:1 and very particularly preferablyin a ratio of 1:1.

The new process is carried out e.g. by a procedure in which thethiophenes to be purified and at least one solvent are brought togetherin any desired sequence, the solvent or solvents, before being broughttogether with the thiophene, or the solution obtained during or afterbringing them together, is or are cooled down to a temperature at whicha mixture of a solid and a liquid forms, the mixture of a solid and aliquid is optionally subsequently stirred and the solid is thenseparated off.

Preferably, the solvent or solvents before being brought together withthe thiophenes, or the solution obtained during or after bringing themtogether is or are cooled down to a temperature which is at least 10°C., preferably at least 20° C. below the melting temperature of the purethiophene to be purified. Cooling particularly preferably takes place to0° C. or lower, very particularly preferably to −15° C. or lower.

The new process can be carried out, for example, by dissolving thethiophenes in the solvent(s) and then cooling this solution down atleast to the extent that the purified thiophene precipitates out orcrystallizes out.

In this procedure, the thiophene can be dissolved in the solvent(s) at atemperature above the melting point of the thiophene. In this context atemperature of between 0° C. and +40° C. is preferred. A temperature ofbetween +15° C. and +25° C. is particularly preferred.

The solution obtained from the solvent and the thiophene is then cooled.The solution is cooled here until the thiophene separates out orcrystallizes out of the solution in the form of a solid. Preferably, thesolution is cooled to a temperature of at least 20° C. below the meltingtemperature of the pure thiophene. Cooling to −15° C. or to atemperature of lower than −15° C. is particularly preferred.

The solution is preferably cooled down at a rate such that the thiophenecrystallizes out within a period of a few minutes to several hours.Cooling down to the desired temperature over a period of approx. onehour is preferred here.

The cooling down can be effected by external cooling or by introductionof an inert cooling medium. The cooling down is preferably achieved byexternal cooling.

During the cooling phase, the thiophene separates out of the solution asa solid, for example in the form of crystals. In this context, the solidobtained can contain the thiophene as the pure substance or can consistof a mixture of the solvent(s) and the thiophene.

Alternatively, the new process can be carried out by a procedure inwhich the liquid thiophene is metered into the already cooled solvent.Solvent mixture or cooled thiophene solution.

In this case the solvent is cooled to a temperature of at least 20° C.below the melting temperature of the pure thiophene. Cooling to −15° C.or a temperature of lower than −15° C. is particularly preferred.

The liquid thiophene is then metered into the cooled solvent—preferablyover a period of a few minutes to several hours. The metering rate is tobe chosen here such that the thiophene does not precipitate out orcrystallize out too rapidly and impurities are thereby also included inthe solid. A metering time of at least 1 hour is preferred. However,metering times of less than one hour may also be sufficient, dependingon the amount of thiophene which must be metered in. The solid obtainedcan also contain the thiophene as the pure substance or consist of amixture of the solvent(s) and the thiophene.

Preferably, the suspension obtained is then subsequently stirred for aperiod of 1 minute up to 5 hours. A subsequent stirring time of approx.three hours is particularly preferred here.

The subsequent stirring is carried out at a temperature of at least 20°C. below the melting temperature of the pure thiophene. A temperature of−15° C. or a temperature of lower than −15° C. is preferred here.

The product which has precipitated out or crystallized out is thenseparated off by known methods. This separating off is preferablycarried out by a filtration. The filtration can be carried out undernormal pressure or under pressure.

The filtration is preferably carried out with the aid of a filter unitwhich can be temperature-controlled, and is preferably carried out suchthat the product to be filtered is present as a solid during thefiltration. The filtration is preferably carried out at a temperature ofbetween 0° C. and −20° C. Preferably, the filtration is carried out at−15° C. or a temperature of lower than −15° C.

Thereafter, the solid obtained can be washed with one or more solvent(s)in order to remove residues of impurities from the filter cake. Polarsolvents are preferably used for this purpose. Alcohols, optionally in amixture with one another and/or with further solvents, are particularlypreferably used. The solid is particularly preferably washed withethanol or methanol or a mixture of these.

In the case where the filter cake is washed to remove impuritiesadhering to the filter cake, it is appropriate to cool the washingagent, i.e. the solvent used for the washing, in order to preventrelatively large amounts of purified thiophene from dissolving in thewashing agent. The washing agent has a temperature below 0° C. duringthe washing. Preferably, the washing agent is cooled down to −15° C. orlower for the washing.

The solid then obtained is warmed to a temperature above the meltingpoint of the thiophene over a period of between 5 minutes and 5 hours.Preferably, the solid is allowed to melt over a period of 1 hour.

After the melting, the molten solid may still contain residues of thesolvent added before the crystallization or residues of the washingagent. These residues can be removed by methods known to the expert,e.g. by simple distillation. The solvent is distilled over during thedistillation. The distillation can be carried out under normal pressureor under reduced pressure. Preferably, it is carried out under reducedpressure at temperatures of between 30° C. and 150° C., preferablybetween 50° C. and 100° C.

The thiophene obtained in this way, which remains as the bottom product,preferably has a purity of at least 99.50%, preferably at least 99.9%,after the solvent has been distilled off completely. For example,thiophenes which have been synthesized using 3,4-dimethoxythiophene orduring the synthesis of which 3,4-dimethoxythiophene is produced as aby-product contain less than 0.05 wt. % of 3,4-dimethoxythiophene afterpurification with the process according to the invention. Such a lowcontent of 3,4-dimethoxythiophene cannot be achieved or can be achievedonly with a very high loss in the yield of the desired thiophene withconventional purification processes, such as e.g. simple distillation.

Thiophenes of such purity are not known. Therefore another subjectmatter of the invention is a thiophene of the general formula (I),

wherein

-   R¹ and R² independently of one another represent hydrogen,    optionally substituted C₁-C₂₀-alkyl groups or C₁-C₂₀-oxyalkyl groups    which are optionally interrupted by 1 to 5 oxygen and/or sulfur    atoms, or together represent an optionally substituted    C₁-C₂₀-dioxyalkylene or C₁-C₂₀-dioxyarylene group,-   characterized in that it has a purity of at least 99.50 wt. %, in    particular of the least 99.9 wt. %.

Particular preferred is a 3,4-ethylene-dioxythiophene with such purity.

Unless mentioned otherwise—all the purity data are data in percent byweight.

The thiophene remaining as the bottom product after the distillation canalso be distilled over to separate off traces of colouring substances.As a rule, a thiophene which is colourless to the eye is obtained bythis means. The distillation of the thiophene is also preferably carriedout under reduced pressure.

By way of example, up to 70%, preferably up to 90%, particularlypreferably up to 95% and very particularly preferably virtually 100% ofthe thiophene employed is obtained in the purified form, depending onthe amount of solvent used in relation to the amount of thiopheneemployed and depending on the temperature during the precipitation and,where appropriate, during the washing. Any remaining portion of thethiophene employed remains dissolved in the mother liquor, i.e. e.g. inthe filtrate separated off during the filtration, or, where appropriate,in the washing agent. Since a recovery, in the purified form, ofvirtually 100% of the thiophene employed is desirable, in a preferredembodiment the purification process can also be carried out by aprocedure in which the mother liquor of a preceding precipitation orcrystallization and/or the washing agent is or are employed again assolvent or together with the solvent in the process for the purificationof further thiophene.

The process according to the invention renders possible the purificationof thiophenes in a simple procedure. The products are moreover obtainedin good yields.

Because of their high purity, the thiophenes purified by the processaccording to the invention are outstandingly suitable for thepreparation of conductive polymers or for the preparation of organicsemiconductors which are suitable e.g. in the production of capacitors,printed circuit boards, antistatic layers, transparent conductivelayers, displays, electrochromic glazing and integrated semiconductorcircuits. These uses are further subject matter of the invention.

The following compounds may be mentioned by way of example as compoundswhich can be purified with the process according to the invention:3,4-ethylenedioxythiophene, 3,4-methylenedioxythiophene;R,S-3,4-(1′-hydroxymethyl)ethylenedioxythiophene,S-3,4-(1′-hydroxymethyl)ethylenedioxythiophene;R-3,4-(1′-hydroxymethyl)ethylenedioxythiophene,3,4-(2′-hydroxy)propylenedioxythiophene;3,4-(1′-methyl)ethylenedioxythiophene,3,4-(3′-tert-butyl)benzodioxythiophene;3,4-(1′-n-hexyl)ethylenedioxythiophene;3,4-(1′-ethyl)ethylenedioxythiophene;3,4-(1′-n-propyl)ethylenedioxythiophene;3,4-(1′-butyl)ethylenedioxythiophene; thieno[3,4-b]-1,4-oxathiine;3,4-ethylenedioxythiophene-1-methyl N-methylcarbamate;3,4-ethylenedioxythiophene-1-methyl N-ethylcarbamate;3,4-ethylenedioxythiophene-1-methyl N-hexylcarbamate;3,4-ethylenedioxythiophene-1-methyl N-phenylcarbamate;3,4-ethylenedioxythiophene-1-methyl N-tolylcarbamate;(3,4-ethylenedioxythiophene-1-methyl) methyl ether;(3,4-ethylenedioxythiophene-1-methyl) ethyl ether;(3,4-ethylenedioxythiophene-1-methyl) propyl ether;(3,4-ethylenedioxythiophene-1-methyl) hexyl ether; 3-hexylthiophene; and3-octylthiophene.

EXAMPLES Example 1 Purification of 3,4-ethylenedioxythiophene

1,800 g 3,4-ethylenedioxythiophene having a purity of 98.4% and acontent of 3,4-dimethoxythiophene of 0.3% and a slightly yellowishcolour were stirred with 2,400 ml ethanol in a sulfonating beaker. Thesolution was cooled down to a temperature of −15° C. by external coolingand stirred at −15° C. for 3 h. The solid formed was separated off withthe aid of a suction filter and washed with ethanol precooled to −15° C.The filter cake was warmed to a temperature of +20° C. In a distillationapparatus comprising a reservoir flask, a distillation bridge and acondensation flask, the solvent was first distilled off under a pressureof 16 hPa at a temperature of 50° C. and 3,4-ethylenedioxythiophene wasthen distilled at a temperature of 90° C. under a pressure of 16 hPa.1,374 g 3,4-ethylenedioxythiophene (76% of theory) were obtained in apurity of 100%. The colourless product no longer contained3,4-dimethoxythiophene.

Example 2 Purification of 3,4-ethylenedioxythiophene

1,800 g 3,4-ethylenedioxythiophene having a purity of 70% and a contentof 3,4-dimethoxythiophene of 0.3% and a dark brown colour were stirredwith 1,800 ml ethanol in a sulfonating beaker. The solution was cooleddown to a temperature of −23° C. by external cooling and stirred at −23°C. for 3 h. The solid formed was separated off with the aid of a suctionfilter and washed with ethanol precooled to −15° C. The filter cakeseparated off was warmed to a temperature of +20° C. In a distillationapparatus comprising a reservoir flask, a distillation bridge and acondensation flask, the solvent was first distilled off under a pressureof 12 hPa at a temperature of 50° C. and 3,4-ethylenedioxythiophene wasthen distilled at a temperature of 90° C. under a pressure of 12 hPa.718 g 3,4-ethylenedioxy-thiophene (55% of theory) were obtained in apurity of 99.2%. The product no longer contained 3,4-dimethoxythiophene.

Although the invention has been described in detail in the foregoing forthe purpose of illustration, it is to be understood that such detail issolely for that purpose and that variations can be made therein by thoseskilled in the art without departing from the spirit and scope of theinvention except as it may be limited by the claims.

1. A process for purifying a thiophene represented by general formula(I),

wherein, R¹ and R² are each selected independently from the groupconsisting of hydrogen, linear or branched C₁-C₂₀-alkyl groups, linearor branched C₁-C₂₀-oxyalkyl groups, linear or branched C₁-C₂₀-oxyalkylgroups which are interrupted by 1 to 5 oxygen atoms, linear or branchedC₁-C₂₀-oxyalkyl groups which are interrupted by 1 to 5 sulphur atoms, afused ring of linear or branched C₁-C₂₀-dioxyalkylene formed by R¹ andR² together, and a fused ring of linear or branched C₁-C₂₀-dioxyaryleneformed by R¹ and R² together, and said thiophene being liquid at roomtemperature, said method being selected from the group consisting of, amethod (I) comprising, (a) providing a first solution comprising saidthiophene and at least one solvent, and (b) cooling said first solutionto a temperature below the melting temperature of said thiophene,thereby precipitating said thiophene as a solid from said firstsolution, and a method (II) comprising, (a) providing a second solutioncomprising at least one solvent and optionally said thiophene, (b)cooling said second solution to a temperature below the meltingtemperature of said thiophene, to form a cooled second solution, and (c)adding said thiophene to said cooled second solution, therebyprecipitating said thiophene as a solid from said cooled secondsolution.
 2. The process of claim 1 wherein said thiophene is a compoundrepresented by general formula (II)

wherein, A is selected from the group consisting of a C₁-C₅-alkyleneradical, a substituted C₁-C₅-alkylene radical, a C₁-C₁₂-arylene radicaland a substituted C₁-C₁₂-arylene radical, the substituted groups of saidsubstituted C₁-C₅-alkylene radical and said substituted C₁-C₁₂-aryleneradical being selected independently from the group consisting ofhalogen, ether, thioether, disulfide, sulfoxide, sulfone, sulfonate,amino, aldehyde, keto, carboxylic acid ester, carboxylic acid,carbonate, carboxylate, cyano, alkylsilane, alkoxysilane andcarboxylamide groups, R is selected, independently for each x, from thegroup consisting of a linear or branched C₁-C₁₈-alkyl radical, aC₅-C₁₂-cycloalkyl radical, a C₆-C₁₄-aryl radical, a C₇-C₁₈-aralkylradical, a C₁-C₄-hydroxyalkyl radical and a hydroxyl radical, xrepresents an integer from 0 to
 8. 3. The process of claim 2 whereinsaid thiophene is a compound represented by the general formula (IIa),

wherein R is, independently for each y, as defined in claim 2 and ydenotes 0, 1, 2, 3 or
 4. 4. The process of claim 3 wherein y is 0 or 1.5. The process of claim 1 wherein said solvent comprises at least onealcohol.
 6. The process of claim 1 wherein the said first solution andsaid second solution are each cooled to a temperature that is at least20° C. below the melting point of said thiophene.
 7. The process ofclaim 1 wherein said method (I) further comprises separating theprecipitated solid thiophene from said first solution at a temperaturethat is at least 20° C. below the melting point of said thiophene, andsaid method (II) further comprises separating the precipitated solidthiophene from said cooled second solution at a temperature that is atleast 20° C. below the melting point of said thiophene.
 8. The processof claim 1 wherein said method (I) further comprises removing residualsolvent from the precipitated solid thiophene by means of distillation,and said method (II) further comprises removing residual solvent fromthe precipitated solid thiophene by means of distillation.
 9. Thepurified thiophene prepared by the process of claim 1 wherein thepurified thiophene has a purity of at least 99.50 wt. %.
 10. A thiophenerepresented by general formula (I),

wherein, R¹ and R² are each selected independently from the groupconsisting of hydrogen, linear or branched C₁-C₂₀-alkyl groups, linearor branched C₁-C₂₀-oxyalkyl groups, linear or branched C₁-C₂₀-oxyalkylgroups which are interrupted by 1 to 5 oxygen atoms, linear or branchedC₁-C₂₀-oxyalkyl groups which are interrupted by 1 to 5 sulphur atoms, afused ring of linear or branched C₁-C₂₀-dioxyalkylene formed by R¹ andR² together, and a fused ring of linear or branched C₁-C₂₀-dioxyaryleneformed by R¹ and R² together, and said thiophene having a purity of atleast 99.50 wt. %.
 11. The thiophene of claim 10 wherein the thiopheneis 3,4-ethylenedioxythiophene.
 12. The thiophene of claim 10 wherein thethiophene has a content of dimethoxythiophene of less than 0.05 wt. %.13. A conductive polymer comprising residues of the thiophene of claim10.
 14. A semiconductor comprising the conductive polymer of claim 13.15. An article of manufacture selected from the group consisting ofcapacitors and printed circuit boards, wherein said article ofmanufacture comprises the conductive polymer of claim 13.